Fundamentals of lubrication - Infineum Insight · 2018-07-12 · Tribology is study of friction, wear and lubrication between surfaces sliding against each other While direct application

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Fundamentals of lubrication



Introduction to Tribology



Outline

At the end of this presentation you will be able to:

– State what the word Tribology means

– List the main regimes of lubrication and state the

conditions under which each tends to occur

– Understand our approach to lubricant performance

testing



Tribology is study of friction, wear and lubrication between

surfaces sliding against each other

While direct application of tribology by Ancient Egyptians is well documented,

Leonardo Da Vinci was the first to enunciate the laws of friction

The word ‘Tribology’ came later and was first coined by David Tabor and Peter

Jost in 1964

Tribology history

Source: WSJ



Tribology solving real world problems

Tribology is about understanding, analysing, predicting and controlling interactions between moving surfaces



Why does tribology matter?

Environment Reliability

Safety

Energy



Multi-disciplinary aspect of tribology

Tribology

Fluids mechanics

Material science

Physical-chemistry

Solid mechanics

o Metallurgy o Coatings o Crystallography

o Rheology o Fluids dynamic

o Stress o Deformation o Motion

o Fluids structure interactions o Surface chemistry o Thermochemistry



Stribeck curve and lubrication regimes



Stribeck curve

Stribeck curve is used to represent friction response of a tribological

contact across different lubrication regimes

Log Viscosity x speed

Load

Boundary lubrication

Mixed lubrication

Elastrohydrodynamic

Hydrodynamic

Min

imu

m film

thic

kn

es

s (h

) Co

eff

icie

nt

of

fric

tio

n (

µ)

h/σ<<1 h/σ≈1 1<h/σ<3 h/σ>>3

σ = composite

surface roughness



The contact is designed such that their relative motion drags (or entrains)

lubricant in between them, forcing the lubricant to high pressures of up to

200 MPa (30,000 psi), large enough to support external loads.

Hydrodynamic lubrication (HD)

Oil Flow

Moving Surface

Pressurised film

creating a 'lift' of the top surface

Bearing Bearing load

Journal

Lubrication gap

Minimum oil film thickness

Hydrodynamic

oil film pressure profile

Lube oil supply (2÷6 bar)

Oil film thickness typically 1 to 100μm



Plain journal bearings

Hydrodynamic lubrication - examples



When all the loading is concentrated over a small contact area.

• High localised stresses cause elastic deformation of the surfaces

• An exponential rise in viscosity of the lubricant as it is squeezed

through the contact

• Thin fluid film is formed due to surface deformation and viscosity

increase

Elastohydrodynamic lubrication (EHL)

Oil film thickness typically 0.1 to 1μm



EHL - examples

Example of a component experiencing

elastohydrodynamic lubrication

Toyota 1HZ timing gear Valve train – cam and lifter experience EHL at cam nose



Oil film thickness typically 1 to 100 nm (roughness dependent)

Mixed lubrication

Enough film to separate

most of the rubbing surfaces

Some contact between the

asperities + reacted film


Enough film to separate

most of the rubbing surfaces

Some contact between the

asperities + reacted film




Boundary lubrication

Insufficient film to

separate the surfaces

Tribo-reacted film on the

contacting asperities



Mixed and boundary lubrication examples

Top Dead Center (TDC)

Bottom Dead Center (BDC)

Mixed/boundary to EHL

Piston rings experience all lubrication

regimes as piston slides from TDC to BDC



Applied tribology and key properties



Functional Requirements of Lubricants

• Keep surfaces separate under all loads, temperatures

and speeds, thus minimising friction and wear.

• Act as a cooling fluid removing the heat produced by

friction or from external sources

• Remain adequately stable in order to guarantee

constant behavior over the forecasted useful life

• Protect surfaces from the attack of aggressive

products formed during operation

• Fulfil detersive and dispersive functions in order to

remove residue and debris that may form during

operation



Parameters to consider in tribology

• Material

– Roughness

– Metallurgy

– Hardness

• Fluid properties

– Viscosity

– Newtonian vs. Non-Newtonian

– Pressure viscosity coefficient

• Contact conditions

– Pressure

– Temperature

– Rubbing part entrainment speed

• Surface active additives



Material properties - examples

Roughness

Metallurgy Hardness

Real example of top ring roughness

discrepancy

Chilled cast iron

Grey cast iron



Fluids Properties - Viscosity

• Dynamic viscosity: resistance to shearing flow

• Kinematic viscosity: flow response to gravity

• Viscosity Index: VI is an empirical parameter that compares kinematic

viscosity of a given oil to the viscosities of two reference oils that have

appreciable difference in sensitivity of viscosity to temperature.

𝜂 = 𝜏

𝛾 𝑆ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 (

𝑁

𝑚2)

𝑆ℎ𝑒𝑎𝑟 𝑟𝑎𝑡𝑒 (𝑠−1) [Pa.s]

𝜗 = 𝜂

𝜌 𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦

𝑑𝑒𝑛𝑠𝑖𝑡𝑦 [𝑚2/𝑠]

300

250

200

150

100

50

0

Kin

em

ati

c V

isc

os

ity,

mm

2/s

Temperature, C 30 40 50 60 70 80 90 100 110

VI > 250

VI ~ 65



Fluids Properties – High pressure

• Viscosity pressure relationship: lubricant viscosity increases with

pressure and this effect is generally greater than the effect of temperature.

– Barus equation is most commonly used to show the relationship:

𝜂𝑝 = 𝜂0𝑒𝛼𝑝

𝜂𝑝 viscosity at pressure ‘p’ [Pa.s]

𝜂0 viscosity at atmospheric pressure [Pa.s]

𝛼 is the pressure viscosity coefficient [m2/N]



Physical Properties of Lubricants

• Viscosity-shear rate relationship: Almost all lubricants behave as non-

Newtonian under high shear rates (~106 s-1 and above), i.e., shear stress

and shear rate are not directly proportional

– Pseudoplastic behavior: during the shearing process the randomly oriented

long molecules tend to align resulting in reduction in apparent viscosity. This is

also referred to as shear thinning.

– Thixotropic behavior: is associated with a loss of consistency of the fluid as the

duration of shear increases. This is also known as shear duration thinning.

Shear rate

Vis

co

sit

y

Newtonian

Shear stress

Sh

ea

r ra

te

Pseudoplastic behavior Thixotropic behavior



Tribology and Infineum



Why study friction and wear in an engine?

• Combustion engine contains many moving metal parts

• Movement between parts can lead to surfaces wearing

away

• Affect durability

• Certain engines are more prone to wear

• As such these troublesome engines tend to be used for

the qualification of a formulated oil



Tribology of an internal combustion engine

Crankshaft

Conrod

Big end bearing

Piston

Valves

Camshaft

Tappet



Levels of tribo-testing

• Level A. Vehicle on- and/or off-road tests

• Level B. Full-size dynamometer test stand (entire vehicle)

• Level C. Full-scale engine tests (engine test cells)

• Level D. Sub-assembly tests (full-scale mating parts)

• Level E. Coupon tests (sub-scale tests, part sections or simple

coupons)

• The complexity and the cost of testing goes up as the testing moves

from Level E to Level A

• Control of operating variables and fundamental learnings increase

as the testing moves from Level A to Level E



Factors determining design of a tribo-test

Designing a tribo-test a correlate performance of materials (metals

and/or lubricants) across various levels of testing requires

consideration of several factors

Mechanical factors surface geometry, design,

relative motion, contact stress,

vibrations

Thermal factors heat generation and

dissipation rate

Third bodies wear debris, contaminants

Material factors composition, processing,

surface treatments

Chemical factors lubricant chemistry, tribo-

chemistry, oxidation, corrosion

Lubrication factors lubrication regime, fluid flow,

film thickness



Bench test rigs (Level E)

Examples of Level E tribo-testing

SRV-5 reciprocating test rig Block-on-ring test rig



Sub-system testing (Level D)

Valve-train test rigs developed by Infineum used for formulation

development and fundamental understanding

Examples of sub-assembly test rigs



Engine testing (Level C)

Examples of engine test installation used for evaluation and

validation of lubricant performance



Vehicle testing (Level B)

Examples of vehicle testing on a dynamometer to evaluate lubricant performance in

real driving condition in a very control and repeatable environment



Vehicle testing (Level A)

Examples of field testing to validate lubricant performance in a very

variable but representative of the end-user utilisation environment



Surface examination tools

5x to 100x 25x to

1000x

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=ERhEPVF8bWtoMM&tbnid=MpdgD69dyhs23M:&ved=0CAUQjRw&url=http://www.dia-m.ru/lab/mikroskopy-pryamye-issledovatelskie/carl-zeiss-4749-axio-scope-a1/&ei=bwC_Ue_WLInM0QX56IGQDA&bvm=bv.47883778,d.d2k&psig=AFQjCNHB1rkRXyhJTnVcsqxP2Nb9lAnN8w&ust=1371558325015126



Surface examination tools: Optical interferometer

Example – HD resolution measurement of a liner

honing marks (HDD engine)

• Zemetrics Ze-scope

• 5x to 50x objective

• Max field of view 3.35mm x 2.58mm

• 1nm vertical resolution

• HD imaging camera

• Fully motorised

• Greater vertical resolution



Surface examination tools: Scanning electron

microscope

Range of magnification: 10x – 1000,000x

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=FCca2ne6wLm4_M&tbnid=hUX9_SN51WiPqM:&ved=0CAUQjRw&url=http://arboretum.harvard.edu/research/policies-and-instrumentation/microscopy-lab/?wpmp_switcher=mobile&ei=kXYoUv2TII-Z0AXA4YHQCg&bvm=bv.51773540,d.ZG4&psig=AFQjCNF8m6v1mhVgX0Tf_Bc2jzZaGo28FA&ust=1378469850751472



Summary

This presentation should have helped you to understand more about:

– What Tribology is and how we use it in developing

new lubricants

– The main lubrication regimes and conditions under

which they occur

– Different levels of tribo-testing

– Understand our approach to lubricant performance

testing and surface analysis techniques



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Fundamentals of lubrication - Infineum Insight · 2018-07-12 · Tribology is study of friction, wear and lubrication between surfaces sliding against each other While direct application